A prominent Atlanta-based law firm hired by Republican lawmakers to defend the federal ban on gay marriage said it was withdrawing from the case amid criticism by advocacy groups, prompting the partner leading the work to quit.

Chairman Robert Hays Jr., with King & Spalding, said the firm chose to divorce itself from the controversy after determining that the decision to take the case wasn't vetted properly.

However, gay rights groups had been pressuring the law firm with plans to protest Tuesday in Atlanta and to make calls to its other clients.

House leaders had earlier picked former Solicitor General Paul Clement, partner in the King & Spalding firm, to fight for the Defense of Marriage Act. The Obama administration has refused to defend the Bush-era law that defines marriage as between one man and one woman.

Clement said he accepted the case believing he had the full backing of his firm and that he's honor-bound to continue, even if that means resigning. He has announced he was moving to the Washington-based firm Bancroft PLLC.

A House committee voted in March to fund the defense of DOMA and officials announced last week that they were retaining Clement at $520 an hour.

Conservative groups have spoken out against the law firm's action. Family Research Council President Tony Perkins said he was shocked that the firm "would rather lose their most brilliant and talented Supreme Court lawyer than confront a smear campaign" by gay rights groups.

Penny Nance, the chief executive officer of Concerned Women for America, likened the decision to "caving to extremists under pressure."

Clement said efforts to make one side of a legal controversy seem less legitimate are a "profound threat to the rule of law."

"Much has been said about being on the wrong side of history. But being on the right or wrong side of history on the merits is a question for the clients," Clement said.